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Star Trailing

Roysten

By Roysten
in Getting Started With Imaging

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Roysten

Roysten

Posted
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Hi Guys

Since we lost Jupiter, Saturn and Mars from easy viewing I've been getting into photographing deep-sky-objects, initially I wasn't really interested but since seeing what other people have done and getting a glimpse of a couple of objects myself I've been inspired! I recently bought an EQ3-2 dual axis motor drive system and have been reading up on polar alignment.

I've attached my best efforts to date on M42, orders of magnitude away from the majestic images I've seen on here but I've been seeing a steady improvement as I've used the motors and improved my technique. I'd like to highlight the star trailing evident in the oblong appearance of the stars in the image.

I don't think I'm imaging long enough for periodic error to creep in (please correct me if I'm wrong!) and the drift seems to not be in the RA axis so I'm guessing it's DEC drift? I'm aware that the EQ3-2 is not really adequate for long exposures of DSOs but the most I've been able to achieve without considerable trailing is an 8 second sub, when I feel like I should be able to achieve a stable frame for maybe up to 20-30 seconds.

I've been polar aligning by eye as I don't have a polarscope and I've been having trouble getting my mount fully aligned before I begin a polar alignment as I'm not 100% sure that I've got the mount fully straightened before aligning. The DEC axis has an arrow that allows me get it lined up nicely at 90º, however the the scale for the RA axis (shown in the other attached image) moves as I move the RA axis unless I lock it in place, so I don't know whether I'm completely straight or not, which I'm assuming can affect the quality of my alignment.

Any advice or comments would be much appreciated!

Thanks, Roysten

Orion 13-1-17.jpg

EQ3-2.jpg

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cjdawson

cjdawson

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The star trailing that you are showing is perfectly normal,  this is what happens as a result of.... PE, imperfect PA, the motor tracking at the wrong speed and changes in the atmosphere.

They'll all be playing a part to a greater of lesser degree.

 

I think you are right that the subs are short enough that PE won't make a big effect and should be safe to ignore.

PA is most likely the cause of your Dec drift.  The drift will happen as the alignment error manifests.  The solution to this part is to perform a drift alignment.  There are various ways of doing it depending on your equipment capabilities (I'm not familier with your mount)

As you say that your star trails are in Dec only, we can rule out any motor tracking issues (for information that would be easily fixed by calibrating the motors, the method of this varies depending on the mount)

The changes in atmosphere.  Well, that's just plain tricky.  The pro's do with active optics which is horrendously expensive.  You can get it, but it's not really needed at the size of scopes we use.

 

Of course, there is one more option to help resolve some of these issues... This is where Autoguiding comes into the equation.  Again I don't know your mount, so don't know if that's a viable idea.

 

Looks to me like you are on the right track to making some great images.

 

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Roysten

Roysten

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Cheers for the response guys!

@Nova2000, my latitude in Hull is 53.75. I don't have a guiding scope just the camera attached for prime focus imaging.

@cjdawson, I have plenty of atmospheric issues all the time, am still waiting for a truly calm, clear night this year! The wandering is fairly consistent so I'd say it's the alignment. Have never performed a drift alignment, sounds like a good next step! I've thought about going full guided, but I probably won't do that with this setup, think I'll wait until I've got a heavier mount. Thanks for the encouragement!

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Roysten

Roysten

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Hi Happy-kat. Yes I'm using a Skywatcher Explorer 150P on an EQ3-2, which I know isn't really ideal for DSO imaging.

The wind is definitely an issue, but the trails are too consistent IMO for it to be due to the wind.

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dph1nm

dph1nm

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On 21/01/2017 at 18:47, Roysten said:

I've been polar aligning by eye as I don't have a polarscope and I've been having trouble getting my mount fully aligned before I begin a polar alignment as I'm not 100% sure that I've got the mount fully straightened before aligning. The DEC axis has an arrow that allows me get it lined up nicely at 90º, however the the scale for the RA axis (shown in the other attached image) moves as I move the RA axis unless I lock it in place, so I don't know whether I'm completely straight or not, which I'm assuming can affect the quality of my alignment.

 

Polar alignment is not affected by the relative positions of the RA and Dec axes - this only affects GOTO alignment. You should polar align first before you do anything else. I would definitely try drift alignment.

NIgelM

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Waldemar

Waldemar

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On 22-1-2017 at 12:11, Roysten said:

Cheers for the response guys!

@Nova2000, my latitude in Hull is 53.75. I don't have a guiding scope just the camera attached for prime focus imaging.

@cjdawson, I have plenty of atmospheric issues all the time, am still waiting for a truly calm, clear night this year! The wandering is fairly consistent so I'd say it's the alignment. Have never performed a drift alignment, sounds like a good next step! I've thought about going full guided, but I probably won't do that with this setup, think I'll wait until I've got a heavier mount. Thanks for the encouragement!

You say your latitude is 53.75. In your picture it shows closer to 60... That may be a cause.
Level your mount as good as you can (I know it is not really neccessary, but it does make things easier) Then set letitude as good as you can, then go for pa. At least you will be close to start with.

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Roysten

Roysten

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Hey Waldemar

You had my worried so I went to double check. It's actually almost bang-on 55. I think the angle I took the image at made it look higher.

55 is still obviously higher than 53.75. Would that 1.25 make a big difference to the quality of the PA? If so then either my finderscope (that I was using to track Polaris) isn't aligned properly or what I was looking at wasn't Polaris!

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kens

kens

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You can work these things out mathematically. Your scope has a focal length of 750mm and your camera has 3.75um pixels. Using one of the many online calculators (or applying the 206.3 factor) you are imaging at a touch over 1 arc-second per pixel. If you zoom in on one of your stars and count pixels you see there is a drift of about 5 pixels. You didn't say but I assume the image of M42 was one of your 8 second images so you are getting a drift of 5 pixels or about 5 arc-seconds over 8 seconds.

Simplistically, you will drift by the amount of PA error over a 6 hour period (a sine function but lets assume linearly for now) so over 6 hours at 5"/8s you would drift some 230 arc-minutes or nearly 4 degrees. That is an indication of how much your PA is out. This is a plausible amount if you are polar aligning by eye.

Imaging at 1"/pixel is challenging and you would want to get your PA within a few arc-minutes even for guided imaging at that scale.

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Waldemar

Waldemar

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Aligning of your finder scope with the telescope is an important issue if you use it for pa. 
 A PoleMaster does a great job, but it is not cheap.
Like is said before, you are out of focus also, which makes it even harder to get a good idea.

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Anne S

Anne S

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Have you tried using Sharpcap to polar align? It has a routine built in. You use your camera attached to the finderscope which gives a wide field of view. The software works out where it's pointing and works out how much to adjust the mount. I'm pretty sure it works even when your finderscope isn't parallel with the mount axis.

Sharpcap is free.

Anne

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kens

kens

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3 hours ago, Waldemar said:

So SharpCap has a platesolve routine?  I did not know that.

And how does it compensate for the misalignment with the mount's axis?

It requires you to rotate the mount in RA by 90 degrees so it can determine the centre of rotation. You then align the centre of rotation with the pole

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